The invention relates to an arrangement for the cooling of electronic components on a board which is embedded into a metallic shield for protection against electromagnetic radiation.
Electronic components, in particular highly integrated circuits, when in operation, generate heat which has to be discharged so that the components are not destroyed. For this purpose, the air which heats up in the housing in which these components are located is continuously replaced by cooler ambient air by means of a fan. This method is not altogether sufficient when the board having the components is also provided with a metallic shield within the housing in order to ensure electromagnetic compatibility (EMC). Such direct shielding is necessary even in the case of metallic housings which also per se give rise to a certain amount of shielding. This alone is not sufficient, however, since, as a rule, a housing has a large number of orifices for feed lines and operating elements, and the electromagnetic radiation can penetrate through these orifices. To be precise, in order to be fully effective, the metallic shield must be sufficiently leaktight, that is to say it should not have any overly large orifices. However, such a leaktight shield arranged directly on the board also has the result that the air between the board and the shield does not participate in the general air exchange in the housing and consequently the heat discharge is reduced.
It has therefore already been proposed to place a heat-conducting cushion between the components to be cooled and the shield, so that the heat generated by the component is transmitted to the shield which, in turn, is cooled by the air stream sweeping past from outside.
The invention is based on the problem of improving in a simple way the cooling of components on a board within a shield.
For this purpose, it is proposed that, in an arrangement for cooling according to the introductory-mentioned type, the shield has at least one air inlet orifice and at least one air outlet orifice, the first-mentioned orifice being connected to a fan via an air guide duct.
With regard to a shield consisting of two decks which run parallel to and at a short distance from the board and which are connected to one another via side walls to form a largely closed box, an optimal and low-friction distribution of the air stream over the board surface is achieved when the air inlet orifice and the air outlet orifice are formed in the side walls.
Components to be cooled are often arranged on both sides of the board. In such a case, it is advantageous if the air guide duct has two part ducts, one part duct leading into the space between one side of the board and the shield and the other part duct leading into the space between the other side of the board and the shield.
The shield often consists of metal plates, the portions of the side wall on one side of the board having tabs which are inserted through holes located near to one another in the board and which are frictionally and positively connected to corresponding tabs of the shield on the other side of the board. Part of the board, in this case, extends beyond the shield. Particularly in this form of construction, it is appropriate for a component which forms the air guide duct to be slipped onto this portion of the board. For this purpose, the component has a slot in which the board is inserted, said board thereby at the same time forming, if appropriate, a partition between the two part ducts.
There are also shields which consist of a metallized plastic. In such a case, it is appropriate for the air guide duct to be formed in one piece with the shield. The advantage of this is that the shield and the air guide duct can be produced in one operation and be placed simultaneously on the board. In an actual version, there is provision for the shield to consist of two shallow metallized half troughs which are arranged on both sides of the board, and for the air guide duct to be formed by two half shells which are in each case connected in one piece to a half trough.
A fan used typically for the cooling of electronic components consist of a frame, in the center of which is arranged a fan motor which drives a fan wheel, the blades of which run in an annular free space between the frame and the fan motor. Such a fan is located directly in front of the entrance of the air guide duct, only part of the free space being covered. The result of this is that that part of the air stream which is sucked through the noncovered part of the free space enters the housing and cools further components there outside the shield. A single fan can thereby be used both for air exchange in the shield and for air exchange in the housing.
That region of the free space which is covered by the entrance of the air guide duct serves for ventilating the space within the shield. So that the two part spaces above and below the board are supplied approximately with the same air quantity, care must be taken to ensure that the overlaps of the cross-sectional surfaces of the entrances of the part ducts with the cross-sectional surface of the free space are of approximately equal size.
Preferably, the fan is fastened in or on an orifice in the wall of a housing which receives the board, together with the shield, and further, in particular mechanically operating components, the free space, not covered by the air guide duct, of the fan serving for cooling these further components.
The invention will be illustrated in more detail below by means of an exemplary embodiment. For this purpose,